1. Field of the Invention
The present invention relates to a magnetic-bubble memory device (hereinafter referred to as a bubble memory) and more particularly to a bubble propagation track defined by elements or patterns of magnetically soft material.
2. Description of the Prior Art
There are various types of bubble memories known in the prior art, the most common one being an in-plane field access type of bubble memory which comprises a thin layer of magnetic material in which magnetic bubbles can be propagated along propagation tracks in response to a magnetic drive field rotating or reorienting cyclically in the plane of the layer of magnetic material.
An in-plane field access type of bubble memory in which the bit period is 8 micrometers (.mu.m) and the capacity is 1 megabit (Mb) has been realized, and a 4 .mu.m period, 4 Mb bubble memory is now being developed.
There are two well-known types of propagation tracks, one being defined by elements or patterns of magnetically soft material such as permalloy and commonly called a "permalloy propagation track" and the other being defined by an ion-implanted pattern and commonly called an "ion-implanted propagation track". A 4 .mu.m period ion-implanted track having superior propagation performance can be easily fabricated and is a very effective means for realizing a 4 .mu.m period, 4 Mb bubble memory. However, superior-performance function gates for a 4 Mb bubble memory having a 4 .mu.m period ion-implanted track, particularly, block-replicate gates for major-minor loop-organized bubble memories are still in the process of development.
Superior-performance function gates for a bubble memory having a permalloy track have already been realized. However, in realizing a 4 .mu.m period permalloy track, there is a gap problem. There are known gap-tolerant permalloy propagation patterns, such as half-disk and asymmetric chevron patterns. However, an allowable gap for a gap-tolerant pattern is, at the most, one-eighth of the period. Therefore, in a 4 .mu.m period permalloy track defined by gap-tolerant patterns, the gaps should have a width of 0.5 .mu.m, which cannot be achieved by present-day photolithographic resolution.
New permalloy propagation patterns, called wide-gap patterns, have been reported by A. H. Bobeck, et al. (EA-1, 3M conference, Atlanta, 1981). In a wide-gap track defined by a wide-gap pattern (explained in detail with reference to the accompanying drawings), superior bubble propagation performance can be obtained with gaps of one-fourth of the period. Therefore, a wide-gap pattern is a very promising means for realizing a 4 .mu.m period, 1 .mu.m gap permalloy bubble propagation track which can be fabricated by present-day photolithography, thus realizing a 4 .mu.m period, 4 Mb bubble memory.
However, in the realization of a 4 Mb bubble memory with a 4 .mu.m period wide-open propagation track, there is a further problem. Conventional function gates in a bubble memory having a permalloy propagation track are defined by conventional patterns, such as half-disk or pickax patterns, and a conventional permalloy track has different magnetic characteristics than does a wide-gap permalloy track (described hereinafter). The bubble propagation performance when such different types of tracks are connected is poor.